Without limiting the scope of the invention, its background is described in connection with the processing of semiconductor silicon wafers, as an example.
Heretofore, in this field, defects on integrated circuit wafers have been created during the formation of oxidation layers at junctions were hydrophobic and hydrophilic areas are required to coexist. On layered integrated circuit wafers, these junctions correspond to the layering of adjacent silicon and silicon dioxide (SiO.sub.2) layers. For example, it is known that watermarks, also known as waterspots or liquid residue defects, form as a result of certain operations in a manufacturing process flow, for example, that of oxidation. Unfortunately, watermarks are not visible under a microscope until the oxidation process has been completed. The formation of watermarks leads to irregularities in the wafer causing wafer, failures and consequently decreased yield.
As a result of these irregularities, deposition of subsequent layers of material can easily result in incomplete coverage, breaks in the deposited material, or voids if the subsequent layer be deposited directly over the irregular surfaces. If the irregularities are not alleviated at each major processing step, the top surface topography of the surface irregularities will tend to become even more irregular, causing further problems as the layers stack up in the further processing of the semiconductor device.
Depending upon the type of material used and the intended purpose, numerous undesirable characteristics are produced when these irregular layers are deposited. Incomplete coverage of an insulating oxide layer can lead to short circuits between metalization layers. Likewise, voids can trap air or processing gases, either contaminating further processing steps, or simply lowering overall device reliability. Sharp points on conductors can result in unusual, undesirable field effects. One problem that is widely recognized in the wafer manufacturing process is that, in general, processing high density circuits over highly irregular structures can lead to very poor yields and device performance.
As mentioned above, watermarks form at certain logpoints in the process flow, and are not visible under a microscope until after an oxidation step has been completed. Watermarks tend to appear in varying sizes and shapes, and lead to a decrease in device yields by blocking a subsequent etching step, by blocking or altering an implant, or by changing the rate of oxidation growth. Finally, watermarks are a source of impurities that lead to irregularities on the wafer surface.
It is known that watermark formation and defects are caused by hydrofluoric acid used in the clean-up step prior to oxidation. Generally, the hydrofluoric acid (HF) clean-up step leads to watermark formation when the HF step is directly followed by an isopropanol vapor drying step in an IPA dryer. It should be noted by one skilled in the art that when reference is made to HF, the HF solution is diluted with deionized water.
Although the mechanism for watermark formation is not fully understood, it is possible that a residual fluorosililic acid residue or a H.sub.2 SiO.sub.3 residue is responsible. Previous methods for eliminating watermark formation have focused on spin drying of the wafers or modifying IPA dryer machine parameters. The centrifugal or spin drying process can greatly reduce watermarks, but adds large numbers of particles to the wafer surface, thereby trading one problem for another. Furthermore, it is unreasonable to expect that a wafer fab will replace a working IPA dryer with a spin dryer to reduce watermarks because of the time, money and labor involved. Similarly, modifying IPA dryer parameters such as transfer time from rinse to dryer, IPA change-out frequency and IPA dry time have been moderately successful at reducing watermark formation but have not eliminated watermark formation.
Alternatively, an intermediate hydrogen peroxide cleaning step has been used following the hydrofluoric acid step prior to the isopropanol vapor dry. Although this additional step reduced watermark formation, watermarks have still been detected on the last part of the wafer which enters the vapor dryer.
Conventionally, the last chemical step prior to the isopropanol vapor dry step is the most critical for eliminating watermarks. Watermark formation has been reduced when a chemical oxide was grown following the hydrofluoric acid clean-up, prior to vapor drying. By performing the hydrofluoric acid clean-up step just prior to vapor drying, a minimal thickness of native oxide is formed on the silicon wafer surface of about 2 angstroms. On the other hand, when a chemical oxide layer is formed by following the two typical cleaning steps, between the hydrofluoric acid and vapor drying steps, the oxide thickness is greater, between 5 and 10 angstroms.
What is needed, therefore, is a method that allows for the formation of oxidation layers having the optimal 5 to 10 angstrom thickness, but without the watermarks formed by current methods. Also needed is a method for cleaning up silicon/silicon oxide layers that decreases the amount of acidic and basic chemicals thereby increasing the efficiency of the wafer manufacturing. A decrease in the usage of these potentially hazardous chemicals, and a concomitant increase in overall processing cleanliness and wafer yield, would increase yield per wafer and reduce the amount of chemicals created during the formation of integrated circuit layers. A decrease in the use of these chemicals results in both a cost savings from the decreased use of chemicals used in the clean-up process, as well as a reduced environmental impact from the discarded volumes of chemicals.